Conventionally, for example, as disclosed in JP 2014-87874 A, a robot system including a three-phase motor and a three-phase inverter is known. In the robot system, the three-phase motor drives each rotation axis of a robot and a three-phase inverter applies an alternating-current (AC) voltage to a stator winding of the motor using an external power source as a power supply source.
Some robot systems include two contactors disposed between an external power source 100 and an inverter 101, as shown in FIG. 9, in order to secure a stop of a robot in case of an occurrence of an abnormality. Specifically, first and second contactors 102a, 102b are disposed between the external power source 100 and a rectifier 103. The rectifier 103 converts an AC voltage output from the external power source 100 into a direct-current (DC) voltage and applies the DC voltage to an inverter 101. The inverter 101 converts the DC voltage into an AC voltage and applies the AC voltage to a motor 104. The first contactor 102a is driven by a first control portion (FIRST CONT.) 105a, and the second contactor 102b is driven by a second control portion (SECOND CONT.) 105b. By the above-described configuration, means for stopping a robot is duplexed, and a stop of the robot in case of an occurrence of an abnormality in the robot system can be secured.
If a configuration in which a contractor is turned off when an abnormality occurs in a robot system is employed, a free run state in which the motor continues rotating by inertia occurs while a power supply from an external power source to a motor is interrupted. In such a case, the robot cannot be stopped quickly at an occurrence of an abnormality in the robot system.